The present invention relates to a power transformer of toroidal design, and more particularly to an improved means for providing high voltage insulation between the primary and secondary windings of such a toroidal power transformer and for maximizing the creepage distance; this distance being measured, for example, along the surface of the insulating material between the windings. Maximizing the creepage distance insures that the insulation material between the windings will maintain its integrity over a long period of time.
In toroidal power transformers, a primary winding (or set of primary windings) is first wound on a toroid shaped core, covering nearly the entire circumference of the core. One or more secondary windings are then wound on top of the primary windings, with a high-voltage insulating layer separating the primary and secondary windings for forming a safety insulation layer capable of withstanding high-voltage insulation testing and high operating potential differences between the primary and secondary windings.
The high-voltage insulating layer customarily comprises layers of polyester film strips wound on top of the primary winding such that each turn of the strip partly overlaps the prior turn. This kind of insulation provides excellent integrity against break through as well as good creepage distance. A problem in achieving a good creepage distance exists, however, were leads to the primary windings penetrate the insulation and the secondary windings.
There are basically two different procedures in use for assuring sufficient creepage distances and overall integrity of insulation around the primary leads in the region where the leads penetrate the insulation. FIG. 1 illustrates one approach to insure high creepage distance between the primary and secondary windings. This approach essentially comprises: after winding the primary and before adding the insulating layer, taping the primary winding in the area where the primary lead wires pass through the insulating layer. Because this taping involves hand operations, this approach results in a toroidal transformer that is difficult and more costly to manufacture. This approach, also is subject to manufacturing variations leading to transformer breakdown due to variations in the creepage distance caused by the hand taping. Moreover, once the secondary winding has been wound, there is no way of assuring that proper taping was accomplished so as to provide sufficient creepage distance.
In winding the primary shown in FIG. 1, the primary winding is begun at any point around the toroidal core, leaving a free portion of the wire for connection to a first lead wire. The wire is wound around the circumference of the toroidal core to a point adjacent the beginning winding, and leaving another free portion of the wire for connection to a second lead wire. The free portions can be self leads, in which case color-coded insulating sleeves are pulled over the primary leads, or separate insulated leads may be soldered on to the free portions of the primary winding. The solder joints can be insulated in any conventional way.
The creepage distance in a transformer wound as described above is achieved by manually placing insulating tape tangentially over the beginning and end wires of the primary winding. This insulating tape is placed underneath the insulated primary leads. Then, the polyester film strips are wound around the primary windings to form the insulating layer as shown. Next, both primary leads are placed in a separate insulating sleeve. This insulating sleeve is then slit at the end nearest the primary winding and the slit ends are taped to the insulating layer. Finally, the secondary windings are wound on top of the insulating layer and on the slit ends of the insulating sleeve. The secondary windings serve to further mechanically hold the insulating sleeve in place.
This method results in a neat-looking transformer, with the primary leads exiting in a radial plane. The disadvantages of this method are mainly that all taping must be done carefully, and inspection is impossible after the secondary winding has been added. Careless taping may pass the initial short time period high leakage testing, but inadequate creepage distance can not be detected. Thus, it is likely that insulation will breakdown at a later time due to inadequate creepage distance.
FIG. 2 illustrates a second approach for assuring high creepage distance between the primary and secondary windings in the region where the leads penetrate the insulation. In FIG. 2, after a first layer of insulating material has been wrapped around the primary windings, an insulation sleeve, with lead wires positioned therein, is folded over and and wrapped under a second layer of insulating material as shown in FIG. 2(a). This approach, while effectively achieving high creepage distance, results in a bulging toroidal transformer having a bump on the perimeter of the primary winding as illustrated in FIG. 2(b).
Thus, improvements in insulating the primary winding lead wire from the secondary winding are necessary to obtain an improved transformer which is easier to manufacture than conventional toroidal transformers, has the appropriate creepage distance, and has a high relability factor.